entrybuffer: remove text in ::deleted-text default handler

entrybuffer: remove text in ::deleted-text default handler

This changes the semantics of ::deleted-text to perform the removal of
text in the default handler. This means, that if you want the old behavior
where your signal handler is called after the text has been deleted, you
should connect with G_CONNECT_AFTER in your signal handler (or use
g_signal_connect_after).

Without this change, there was never a way to get the deleted-text before
the operation had completed and this is necessary to provide undo support
to the text widgets.